Cable TV (CATV) signals are traditionally broadcast to subscribers through co-axial cables. In recent years, it has become increasingly desirable to use passive optical networks (PONs) in place of the traditional copper co-axial cable infrastructure. In such cases, the conventional electrical CATV signal is typically modulated onto a narrow-band optical carrier using a transmitter of the type illustrated in FIG. 1a. As may be seen in FIG. 1a, the transmitter 2 comprises a laser 4 optically coupled to an external modulator 6. The laser 4 (which may, for example, be provided as a conventional Transmission Optical Sub-Assembly (TOSA)) outputs a narrow-band continuous wave (CW) light 8 having a desired wavelength and optical power. The external modulator 6 (which may, for example, be provided as a variable optical attenuator or a Mach-Zehnder interferometer) receives the electrical CATV signal 10, and imposes a corresponding amplitude (or phase) modulation on the CW light 8 output by the laser 4. The resulting modulated optical CATV signal 12 can then be transmitted to subscribers through an optical fibre network (not shown) such as a passive optical network (PON).
In some cases, it is desirable to generate modulated optical CATV signals 12 on multiple wavelengths. This is typically accomplished by duplicating the arrangement of FIG. 1a for each wavelength. Thus, in the transmitter 14 of FIG. 1b, multiple single-wavelength transmitters 2 are used, in parallel, to generate respective optical CATV signals 12 on the desired wavelengths. If desired, an optical MUX 16 may be used to multiplex the optical CATV signals 12 into a WDM CATV signal 18, which may also be referred to as an optical CATV comb. This arrangement can be useful for coupling the optical CATV signals 12 into a PON, particularly in cases where the CATV transmitter is provisioned separately from the Optical Line Terminal (OLT) equipment of the PON. A limitation of this approach is that the use of multiple parallel transmitters 2, each having its own external modulator 6, is expensive.
A less costly arrangement can be constructed by directly driving each laser 4 with the electrical CATV signal 10. With this arrangement, each laser 4 outputs its respective optical CATV signal 12 directly rather than a CW light 8, so that an external modulator 6 within each transmitter 2 is not needed. However, because the frequency and amplitude responses of a laser diode are coupled, driving a laser 4 with the CATV signal 10 produces CATV signal 12 having excursion of both amplitude and frequency (wavelength). As a result, optical CATV signals 12 generated by direct modulation of the lasers 4 tend to suffer from greater distortion than those generated by an external modulator 6.
Techniques that overcome the above-noted deficiencies of the prior art remain highly desirable.